The synchronous motors (hereinafter referred to as SM) with permanent magnets have a variety of advantages, which allows to distinguish them among all other electrical machines. In particular, a stability of a maintained speed constitutes one of the advantages, which is a paramount requirement for various technological processes, among which, a production of stratum fluids by means of the electric submersible pump units.
The electric submersible pump units, purposed for production of borehole fluids, comprising actuation from the synchronous electric motor of a rotational or progressive action, are currently very common.
A control principle of the mentioned SM is based on an implementation of a vector control of a torque of a moving part of the electric motor, performed by means of an intelligent system of controlling sources of alternate current or voltage, usually by implementing pulse width modulation (hereinafter referred to as PWM).
Various vector control methods have been implemented in the literature and in practice, most of them are based on determining values of a position of an angle of the SM rotor, providing a distinction for a control comprising utilization of rotor position sensors and a sensorless control.
Hall sensors integrated into the electric motor are usually utilized as sensors of the rotor position, which allows to determine the angle with an accuracy of ±30 degrees. Such a control provides a certain number of positions taken by a current vector of a stator for one electrical period, as a result the torque pulsations occur at an output.
The sensorless control is based on utilization of observer units, tracking a back electromotive force (hereinafter referred to as EMF) during a rotation of the motor. However, this system is ineffective at low speeds of the motor, when the back EMF has a low amplitude, that is difficult to distinguish it from a noise interference, therefore this method is not suitable for determining the position of the motor rotor at low speeds.
A common disadvantage of all the SM vector control systems under consideration is a low operation speed of torque control, associated with a presence of an inertia torque of the motor rotation.
A direct torque control (hereinafter referred to as DTC) constitutes an alternative to vector control, while a selection of a corresponding voltage vector depending on a vector position of the rotor magnetic flux and on a difference between a given and an actual operating torque is the main principle of the DTC. The direct torque control has such advantages as aimed rate of torque change response and a high electromagnetic torque at low speeds. However, the operation of the DTC electric drive is accompanied by high pulsations of the electromagnetic torque, especially at low speeds, and an implementation of this method requires significant computational operations, which inhibits a control speed to some extent.
Application for an invention US 20170126160 dated Apr. 5, 2017 sets out a method for controlling a synchronous electric motor with permanent magnets, that comprises measuring output currents supplied to the motor, converting them into Id, Iq currents within d-q rotating coordinate system, determining a required Ud voltage depending on a difference between the measured Id current and the set Id value. Also this method comprises determining a required Uq voltage depending on a difference between a measured Iq current and a set Iq value, determining an angle of an inverter depending on the required Ud voltage and the required Uq voltage, determining the required voltage on a bus of a communication with the motor depending on the required Ud voltage and the required Uq voltage, determining a converter switching angle depending on a difference between a required bus voltage and a measured bus voltage, controlling the converter of the electric drive depending on the converter switching angle, creating the required voltage on the DC bus linked between the converter and the inverter of the electric drive and, consequently, controlling an operation of the inverter depending on the angle of the inverter and forming a six-pulse output signal in a form of a voltage applied to the motor without using a pulse-width modulation.
The described method is based on the sensorless determination of the rotor position, conducted by means of the observer unit, which leads to the above mentioned disadvantages associated with its operation. Also, it is commonly known that, the absence of the pulse-width modulation allows to avoid utilization of output voltage filters, but at the same time leads to a significant decrease in an energy efficiency of the system.